Fixed a bug in which concatenated XFMs were being overwritten.

Specifically, I was creating paths for xfmjoin'ed XFMs such as registrations/atlas/target/nl.xfm
and so the concatenated XFMs with different templates were overwriting one another.  I've fixed
things now so that the xfmjoin'ed path is registrations/atlas/template/target/nl.xfm.  Phew